1. Field of the Invention
This invention relates to a vibration wave motor in which a travelling vibration wave is generated in a vibration member by a voltage being applied to an electro-mechanical energy conversion element and relative movement is caused between said vibration member and a member which is in contact therewith by friction driving, and in particular to a vibration wave motor of the high output type.
2. Related Background Art
A vibration wave motor according to the prior art, particularly a vibration wave motor of the high output type, is formed by a group of thin annular piezo-electric elements, which secured, for example, to the back of an annular vibration member substrate formed of stainless steel. A super-hard material composed of tungsten carbide and cobalt is thermally sprayed onto the surface of the annular vibration member substrate and is polished to thereby form a hard sliding surface. On the other hand, as a member contacting therewith, a sliding member formed into a toughened type composite resin layer by causing a toughening material such as carbon fiber to be contained in a thermoplastic resin having a glass transition point of 100.degree. C. or higher, specifically, heat-resisting resin such as polyimide (PI), polyamideimide (PAI), polyetherimide (PEI), polyether ether ketone (PEEK), polyethersulfone (PES), polyaryrate (PAR), polysulfone (PSF) or aromatic polyamide, is secured to a back-up member of an aluminum alloy or the like. The vibration member and the member contacting therewith are moved relative to each other in a friction driven fashion by a travelling vibration wave generated in the vibration member.
The relative movement of the vibration member and the member contacting therewith may be such that either of them is fixed or movable. However, in the following description herein, for simplicity of description, the vibration member is shown as being fixed and the member contacting therewith is shown as being movable. Accordingly the latter member will hereinafter be referred to as the "movable member".
Now, the reason why in the above-described vibration wave motor according to the prior art, a sliding member formed of thermoplastic resin having a glass transition point of 100.degree. C. or higher as the base material is used as the toughened type composite resin layer forming a part of the movable member is that such heat-resisting resin is small in temperature dependency as the property of matter and is free from torque-down phenomenon attributable to the softening of the resin material even for the temperature rise during the driving of the motor and can stabilize the performance and accuracy of the motor.
Also, the reason why a toughening material such as carbon fiber is combined with said resin material is: firstly, to ensure the properties of the sliding surface of the movable member to be always stable relative to the sliding surface of the superhard material of the vibration member formed, for example, of tungsten carbide and cobalt and, moreover, to ensure sufficient wear resistance during a long time of driving as well; secondly, to make the value of a property of matter such as the elastic modulus or the hardness of the sliding member great and improve the performance of the motor, such as its output; and, thirdly, to improve the heat conductivity of the sliding member and improve the performance of the motor, such as its efficiency.
As described above, in the vibration wave motor, the toughened type composite resin composed of heat-resisting thermoplastic resin having a glass transition point of 100.degree. C. or higher and filled with carbon fiber is used for the sliding member providing the sliding surface of the movable member, whereby the performance and accuracy of the motor are stable even during a temperature rise caused by the driving of the motor, and the wear resistance is sufficient relative to the superhard material forming the sliding surface of the vibration member even when the motor is driven for a long time and further, the performance of the motor, such as output and efficiency, exhibit high values.
However, if the sliding surface of the composite resin layer of the movable member, which is formed of heat-resisting thermoplastic resin toughened by said carbon fiber, is brought into pressure contact with the hard sliding surface of the vibration member, which is formed of the super-hard material, when driving is started, for example, at 4 kg.cm and 100 rpm as the conditions of rated operation, the "waviness" or the like of torque may sometimes pose a problem with the temperature rise of the frictional sliding surface, and it has been found that there are further points to be improved for further improvements in the performance and accuracy of the motor.
Also, in the rated continuous operation, there is torque irregularity of the order of 5% for the rated torque value, and further improvement has been desired.
Furthermore, although there is no problem when the load torque is great, there has been the problem that the phenomenon of so-called "echo" based on sliding friction is caused by driving during no-load or low load.